1. Field of the Invention
The present invention relates to a light irradiation device and a method of pattern light irradiation, and further relates to a three-dimensional shape measuring device using the pattern light irradiation device.
2. Description of the Related Art
In a three-dimensional shape measuring device, there is one in which a light pattern is projected to a physical body to be a target object and an irradiation pattern is analyzed using a phase analysis method such as a fringe analysis method, thereby obtaining three-dimensional information (height information) of the object. To describe such a device in more detail, first, an image of a pattern forming plate is projected to a target object by irradiating light toward the target object from a light source via a pattern forming plate. Then, the target object in which the image of the pattern forming plate is projected is imaged to obtain an image. Next, a pattern of the image of the pattern forming plate in the image is compared with a pattern when the target object is not set (that is, when there is only a standard plane) and height information is calculated on the basis of the amount of pattern deviation (the amount of phase displacement) generated by setting the target object.
In this case, a sinusoidal wave pattern is mainly used as a pattern to be projected to a measurement physical body. This is because that only a boundary portion between bright and dark sections can be used for height calculation in the case of using a rectangular wave pattern; whereas, in the case of using a sinusoidal wave pattern, height can be calculated on the basis of any part of the wave, thereby improving resolution.
An example of the measuring method of the three-dimensional shape measuring device will be described referring to FIG. 11 to FIG. 13. FIG. 11A and FIG. 11B show an example of a target object, each showing a view seen from the upper surface and side surface of the target object. Irradiation of light of a sinusoidal wave pattern onto target object is shown on the left in FIG. 12. In addition, in the drawing, a fringe pattern is depicted in black or white binary without depicting a gradation portion; however, an actual fringe pattern includes gradation having a gradient of luminance.
As shown in the left view of FIG. 12, when the light of a sinusoidal wave pattern is irradiated onto a physical body having height, the sinusoidal fringe pattern produced varies in response to the height of the physical body. The right view shown in FIG. 12 is a graph showing a relationship between a horizontal position in an image and the luminance value of the irradiation pattern in a standard plane in which the physical body is not present and the luminance value of the irradiation pattern of the physical body. As shown in the same drawing, a phase deviation is generated in the sinusoidal wave pattern projected onto a physical body having height. The height can be calculated from the amount of the phase deviation, based on the triangulation principle shown in FIG. 13.
In the pattern light irradiation device included in a three-dimensional shape measuring device, various methods are used in order to irradiate light of a sinusoidal wave pattern. These methods are outlined briefly hereafter. In a first method, a sinusoidal contrasting density pattern for projecting the sinusoidal wave pattern is formed. In this method, the gradation (contrasting density) is formed on the film so that light of the sinusoidal wave pattern is formed. Exposing the film to or using an inkjet printer to print onto the film photosensitive particles such as silver halide results in the formation of the contrasting density on the film.
Furthermore a second method involves using a liquid crystal projector to irradiate a sinusoidal wave shaped light pattern. In this method, the sinusoidal contrasting density pattern is formed by liquid crystal elements and the contrasting density pattern is projected to a target object by the projector. Controlling and adjusting the transmissivity of individual liquid crystal elements results in a given contrasting density pattern.
A third method involves irradiating a diffraction grating having microscopic slits. In this method, diffraction of light is induced by appropriately adjusting the slit width and pitch and interference effects of the diffracted light form light of a sinusoidal wave.
Furthermore, in a fourth method light, in which amplitude (luminance) modulation is generated with time by a modulating signal, is scanned onto a target object. In this method, amplitude modulation is generated so that a relationship between time and luminance is represented as a sine function and the light is scanned onto the target object, thereby irradiating a sinusoidal wave pattern onto the target object.
The following method is disclosed in Japanese Patent Application Laid-Open No. 8-313209, published on Nov. 29, 1996, hereinafter JPA 8-313209. A position measurement device using a micro lens array is described. The position measurement device includes a light source, a scattering plate for scattering light from the light source, a slit mask for selectively passing the light transmitted through the scattering plate, and a lens array disposed in front of the slit mask. In the position measurement device, the slit and the lens array are combined, whereby each lens of the lens array functions as a pseudo light source. Then, lens focus of the lens array is adjusted, so that lights from neighboring lenses overlap to form light of a pseudo sinusoidal wave pattern.
In recent years, the above-mentioned three-dimensional shape measuring device is often used, for example, inspection of a semiconductor packaging substrate. This is for inspecting the incorrect mounting of a component or a soldering defect by measuring height in respective portions of the substrate. In this case, to increase accuracy of the inspection, high-resolution in μm order is required; however, a pitch of the sinusoidal wave pattern needs to be small for increasing resolution. For example, consider a case in which a sinusoidal wave pattern having 200 pitches per one visual field is projected by the pattern forming plate, which is made of a film formed with the above-mentioned contrasting density pattern. Contrasting density needs to be expressed by dots made of photosensitive particles or printing particles in order to form the sinusoidal contrasting density pattern on the film. For example, in the case when the size of one particle is assumed to be 5 μm and contrasting density used for the sinusoidal wave pattern is expressed by a 256 gradation (8-bit gradation), the size of the film needed for one pitch of the sinusoidal wave is expressed as follows:5(μm)×256(gradation)×2=2560(μm)Then, the size of the film required in order to make this for 200 pitches is calculated as:2560(μm)×200(pitch)=512(mm).
In other words, light passing through the film needs to be converged by a lens having not less than 512 mm in image circle. Such a lens is considerably large and very expensive, as compared with a lens having approximately 43 mm in image circle for use in normal 35 mm film cameras. Consequently, the cost of the pattern light irradiation device increases. This remains a problem even if liquid crystal elements are used to form a sinusoidal contrasting density pattern.
Furthermore, in the method of irradiating light of the sinusoidal wave pattern using the diffraction grating, diffraction phenomenon of light is used and therefore a single wavelength light source is required. That is, a special light source is required and therefore cost of the pattern light irradiation device increases and reduction in size is difficult.
Meanwhile, for the method in which light, in which amplitude modulation is generated with time by a modulating signal, is scanned on a target object is not suitable for analyzing the imaged image. The reason is as follows. Only a part of the sinusoidal wave pattern is imaged in the image taken at a certain moment of the target object and consequently height over the whole of the visual field cannot be calculated from the image obtained by one imaging operation.
Furthermore, in the method using a micro lens array described in JPA 8-313209, when the light source is not of a single wavelength, each lens used in the micro lens array is limited to a single plate (single lens) configuration and therefore focal length changes in response to the wavelength of the light from the light source and the effect of chromatic aberration remains. Thereby, the single wavelength light source is required. Consequently, there arises a problem as in the case of using the diffraction grating.
Further, in the method of using a micro lens array, there is also a problem in it is difficult to change the shape of the irradiation pattern. In the measurement of a three-dimensional shape, in order to change the height resolution the pitch of the sinusoidal wave pattern needs to be changed. Furthermore, in the case of imaging the whole or a part of the physical object to perform pattern matching with a template image together with measurement of the three-dimensional shape, light which is not pattern light but has uniform luminance, that is, light without a pattern needs to be irradiated. When shape of the pattern needs to be changed as described, the entire micro lens array must be replaced. Much cost is required for replacement of the entire micro lens array and therefore changing the shape of the irradiation pattern can prove to be difficult.